1. Field of the Invention
The present invention relates to a liquid jetting apparatus for intermittently jetting fine liquid particles, such as is typically seen in ink jet printer heads, and also to a method for making the liquid jetting apparatus.
2. Description of the Related Art
The proliferation of the ink jet printer, as a printer which can implement color printing at low cost, has developed rapidly in recent years. The performance of such ink jet printers is determined by the printer head, which comprises a liquid jetting apparatus that intermittently jets fine liquid particles. There is a demand for ink jet printers that can perform high-resolution printing, multi-gradation printing, and high-speed printing at low cost.
In order to achieve high-resolution printing, the ink particle size for each dot must be made small (small dot diameter). This means that it is necessary to precision-machine the jet tip units that jet the ink. In order to print at 600 dpi, for instance, the dot diameter must be made 40.mu. or smaller, making it necessary to precision-machine the jet tips to 40.mu. or smaller.
In order to achieve multi-gradation printing, the following properties are required in the liquid jetting apparatus. Firstly, the ink particle size and shape must be precisely controlled (dot control). This means that it is necessary to jet the ink with good controllability, that is, to stably jet the requisite amount of ink, using stable jetting means. This must be explained in greater detail. In general, two types of gradation control are employed, namely area modulation and dot modulation. Area modulation is a method of expressing different shades by thinning out the dots being recorded, that is, by lowering the dot recording density. Substantially, then, this is not really high-resolution, multi-gradation printing. Dot modulation, on the other hand, is a method of expressing different shades by altering the quantity of ink being jetted. In order to realize 16-gradation printing in one color using dot modulation, for example, the ink volume must be controlled to within about 5%. Currently, however, due to the difficulty of jetting ink with good controllability, only 2 to 4 gradations per dot can be achieved, which is very low.
In order to realize high-speed printing, the following performance is required in the liquid jetting apparatus. To begin with, it must be possible to jet the ink intermittently in a short time interval. Printing one A4 page at 600 dpi, for example, requires between 30 and 40 million dots. If this is to be done in 1 minute, requiring printing at the speed of 500,000 dots per second or higher, the liquid jetting apparatus must be capable of intermittent jetting at the high speed of 30 million dots per minute or faster. Secondly, many liquid jet nozzles must be lined up abreast of each other (multiple nozzle implementation). In order to effect multiple nozzle implementation, one must machine and align the nozzles with high precision, while keeping costs from getting out of hand and preventing the elements from becoming too large.
Apparatuses that jet ink and other liquids with good precision are used not only in ink jet printers but also in production lines for marking or illustrating industrial products, for liquid drug coating, and other applications. Thus high-speed high-precision liquid jetting apparatuses are also demanded in other fields.
A representative example of an ink jet printer head will now be described as a conventional liquid jetting apparatus. Ink jet printer heads can be broadly classified into thermal heads and piezoelectric heads. The principle of the thermal head is explained first, with reference to FIG. 22. FIG. 22 is a cross-sectional view of the head unit. An ink reservoir 803 is sandwiched into the space between a plate 801 and a plate 802, and a heater 804 is provided between the ink reservoir 803 and the plate 801. Ink is supplied naturally from the supply side (ink injection port 805) of the ink reservoir by capillary action. By passing a current through the heater 804, the ink is quick-boiled, whereupon the ink at the tip is jetted out from the jet nozzle 806.
An example of a piezoelectric ink jet printer head is now described with reference to FIG. 23, which is a cross-sectional view of such a head. Item 811 is a monomorph piezoelectric drive element which is comprised of a stationary plate 813 and a piezoelectric ceramic material 812 provided with electrodes on its upper and lower surfaces. An ink flow path (reservoir) 818, ink injection port 815, and ink jet nozzle 816 are formed by laminating a bonding medium 817 onto a plate 814. The method of forming the monomorph piezoelectric drive element 811 is to print and bake on electrodes consisting of laminated sintered bodies made of zirconia, then print and bake thereon a paste of lead zirconate titanate (PZT) to provide the piezoelectric ceramic, and finally to print and bake thereon an upper electrode. The flow path (reservoir) unit is made by laminating and bonding three layers of a metal sheet with intervening resin bonding films in a structure that provides both a nozzle and an ink supply flow path. The ink is supplied by capillary action through the ink flow path provided in the nozzle unit to the ink drive unit and nozzle unit. Ink jetting is effected from the jet aperture 816 by the ink in the ink flow path unit (reservoir) being put under pressure by the monomorph piezoelectric drive element. To facilitate high-speed printing, the actual head structure has a plurality of the elements described above arranged in a line.
With the conventional thermal and piezoelectric liquid jetting apparatuses, however, the following problems are encountered when seeking to implement high-resolution, multi-gradation, high-speed printing at low cost.
The conventional thermal liquid jetting apparatus has a simple structure which affords the advantage of low-cost production. However, because liquid quick-boiling is utilized to jet the liquid, it is extremely difficult to control both the volume of liquid vaporized and the volume of liquid jetted during the printing of one dot, rendering high-resolution, multi-gradation printing very problematic. Also, heating and cooling are repeated every time the liquid is jetted, so that time is required to raise and lower the temperature of the heater by amounts corresponding to the heat capacity of the gas or liquid that support the heater. Thus high-speed printing is very difficult. Furthermore, since heating and cooling are repeated at high speed, the base unit and heater are damaged by thermal shock, resulting in short useful life.
The piezoelectric liquid jetting apparatus, on the other hand, uses a piezoelectric drive element to push out the liquid. Compared to the thermal type, it can make instantaneous deformations, shorten the time required for repeated liquid jetting, and facilitates jet volume control. However, in order to jet finer liquid particles and to accurately control jet volume, the liquid reservoir and nozzle units have to be formed with a precision commensurable with the desired particle diameter, the piezoelectric drive must be highly controllable, and reproducibility must be good. In conventional piezoelectric liquid jetting apparatuses, holes for the flow path are made in zirconia green sheets, and these sheets are laminated to form the flow paths, making it very difficult to form flow paths of very high precision. But the problems are not limited to the precision with which holes can be made. The flow path shape is deformed in the press during the lamination process, and contraction and flexure occur during the baking process. In forming the monomorph piezoelectric drive elements, moreover, a different piezoelectric ceramic than the flow path material is laminated and baked at high temperature, so the deformation is even greater. The larger the area affected by these baking deformations, the greater the error, making it very difficult to achieve dimensional precision in inter-nozzle pitch, etc., when implementing multiple nozzles to handle high-speed printing. This limits the area which can be handled, and the number of nozzles that can be implemented. Also, when the piezoelectric drive units, liquid reservoirs, and jet units are made as separate components, and bonded together with resin film, faulty positioning, the thickness of the bonding layers, and irregularities in the bonding condition result in subtle differences in the conditions of the flow paths and, hence, in variation in jet volume. Making the piezoelectric drive units, liquid reservoirs, and jet units as separate components requires high-precision positioning, making assembly operations difficult, and leading to higher assembly costs.
In addition, piezoelectric ceramics exhibit non-linear drive characteristics relative to the applied voltage, making high-precision control difficult. Furthermore, since hysteresis is exhibited, variations in jet volume occur, depending on pulse history. In addition, because the material used has a Curie point of about 300.degree., at which polarization processing is easy, the polarization state gradually changes during operation due to auto-exothermic behavior and the effects of the applied voltage. When multiple nozzles are implemented, the characteristics of their respective piezoelectric elements will exhibit variation depending on their composition, sintering conditions, and polarization process conditions.
In the liquid jetting apparatus, moreover, the volume of liquid that must be pressed out in one jetting is the combined capacity of the piezoelectric drive unit and the liquid reservoir (including nozzle unit). In cases where it is difficult to achieve finer implementation, as with the prior art, a large volume of liquid must be pressed out, compared to the volume of the minute liquid particles that it is desired to jet, making high-speed intermittent jetting very difficult.
The present invention resolves the problems described in the foregoing. It achieves high-resolution printing by reducing the size of the liquid particles jetted to the micron order (smaller dot diameters) and by precisely controlling liquid jet volume (dot diameter control). It also achieves multi-gradation printing by enhancing the reproducibility of liquid jet volume (dot reproducibility) and by precision dot diameter control. In addition, it achieves a high-speed printing by jetting liquid intermittently at short time intervals (high-speed jetting) and by increasing the number of nozzles provided in the apparatus (multiple nozzle implementation). An object of the present invention, moreover, is to provide an inexpensive liquid jetting apparatus that exhibits these features together with long useful life. By employing the liquid jetting apparatus of the present invention, high-resolution, multi-gradation, high-speed printing can be realized at low cost.